1. Field of the Invention
The present invention relates to magnetostatic-wave devices, and more particularly, to a magnetostatic-wave device used for a signal-to-noise enhancer.
2. Description of the Related Art
FIG. 11 is a perspective view of a conventional magnetostatic-wave device 1. In the magnetostatic-wave device 1, first electrically conductive lines 3 parallel to each other are formed on a dielectric substrate 2, and a magnetostatic-wave device 4 is disposed on the lines 3. The magnetostatic-wave device 4 includes a gadolinium-gallium-garnet (GGG) substrate 5 and yttrium-iron-garnet (YIG) thin films 6a and 6b serving as ferri-magnetic substrates formed on both main surfaces of the substrate 5. Therefore, the first electrically conductive lines 3 are in contact with the YIG thin film 6a. On the YIG thin film 6b, second electrically conductive lines 7 which are parallel to each other are formed. The first electrically conductive lines 3 are connected to the second electrically conductive lines 7 with bonding wire 8 to form one electrically conductive line having an input end and an output end.
As shown in FIG. 12, the first electrically conductive lines 3 may be connected to the second electrically conductive lines 7 by bonding wire 8 being crossed to form two independent, electrically conductive lines.
In the magnetostatic-wave device 1, when a high-frequency signal is input to the input end while a DC magnetic field Ho is applied in parallel to the first and second electrically conductive lines 3 and 7, a high-frequency magnetic field is generated around the electrically conductive lines 3 and 7, and a magnetostatic wave is excited in the YIG thin films 6a and 6b. When an input signal has a low power, since most of the power is converted to a magnetostatic wave, no signal is output from the output end. Since the excitation of a magnetostatic wave is saturated when the power of an input signal reaches a certain level, if an input signal has a high power, a noise component in the signal, which has a low power, is not output but only the signal component thereof, which has a high power, is output. This means that the magnetostatic-wave device 1 operates as an S/N enhancer.
FIG. 13 shows a dispersion curve indicating the relationship between the propagation constant 2.pi./.lambda. (.lambda.: wavelength) and the frequency of a magnetostatic wave (MSW) in such a magnetostatic-wave device. When a DC magnetic field is applied in parallel to the YIG thin films, a magnetostatic surface wave (MSSW) in a uniform mode which propagates in the width direction of the YIG thin films, a magnetostatic surface wave (MSSW) in a high-order mode which propagates in the width direction of the YIG thin films, and a magnetostatic forward volume wave (MSFVW) are excited. These waves include infinite high-order modes. To simplify the descriptions thereof, only the zero-th order mode and the first-order mode are shown. "Applications of Magnetostatic Wave To Microwave Devices", Makoto Tsutsumi, MWE '92 Microwave Workshop Digest, explains an MSW and an MSSW.
In the magnetostatic-wave devices shown in FIG. 11 and FIG. 12, the minimum wavelength .lambda..sub.min of a magnetostatic wave which can be excited is proportional to the interval between the parallel electrically conductive lines 3 or that between the parallel electrically conductive lines 7. As the interval between electrically conductive lines is decreased, .lambda..sub.min becomes smaller and the propagation constant 2.pi./.lambda..sub.min becomes larger. In this case, it is found from FIG. 13 that the frequency range of a magnetostatic wave which can be excited increases. Since the interval between electrically conductive lines has a lower limit, only a magnetostatic wave having a frequency, for example, in the range indicated by a thick line in FIG. 13 can be excited in a conventional apparatus. In other words, only noise components having frequencies in a limited range can be removed from an input signal. As shown in FIG. 14, noise components are greatly attenuated at discrete frequency ranges, and a frequency range in which noise can be removed becomes narrow. FIG. 15 is a graph showing an experimental result of an insertion loss-frequency characteristic of the magnetostatic-wave device shown in FIG. 11. The input power was set to -40 dBm, which is sufficiently lower than the saturation level. It is found from FIG. 15 that the frequency range width in which an insertion loss of 5 dB or more was obtained is 20 MHz. In addition to the foregoing problems, the wire used for connecting the first and second electrically conductive lines is required to be crossed and to have no electrical connection in the magnetostatic-wave device shown in FIG. 12, and machining therefore is difficult.